Electrical precipitator control system



June 26, 1956 H. J. HALL ELECTRICAL PRECIPITATOR CONTROL SYSTEM 3 Sheets-Sheet. 1

Filed March 4, 1954 CONTROL SYSTEM I I I I I I llllllllll I I I I I l I l l I l I l L TO GRID AMP TUBE TO SELECTOR SWITCH IO INVENTOR HERBERT J. HALL ATTORNEY June 26, 1956 H. J. HALL 2,752,000

ELECTRICAL PRECIPITATOR CONTROL SYSTEM Filed March 4, 1954 3 Sheets-Sheet 2 PRIMARY 7o VOLTAGE WAVE FORM ACROSS I TRANSFORMER 7 3.2- g in.

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COMBINED VOLTAGE WAVE FORM ACROSS POTENTIOMETER I2, SWITCH IO IN POSITION 3 IN POSITION 2. F: g E e VOLTAGE WAVE FORM ACROSS POTENTIOMETER POSITION I INVENTOR HERBERT J .HALL

ao l l R y ATTORNEY June 26, 1956 H. J. HALL 2,752,000

ELECTRICAL PRECIPITATOR CONTROL SYSTEM Filed March 4, 1954 3 Sheets-Sheet 3 SPIKE DUE TO SPARK. AMPLITUDE T I m I TRANSIENT VOLTAGE TIME TYPICAL VOLTAGE WAVE FORM ACROSS RESISTOR 2O OR24.

HIGH I 1 VOLTAGE l RECTIFIER I L l J L 36 i A I I E I l l I I I I I l l DWFERENTIAL ELECTRONIC SPARK MAGNETIC RATE INTEGRATOR AMPLIFIER INVENTOR 32 HERBERT .1. HALL BY fw/ f w ATTORNEY United States Patent ELECTRICAL PRECIPITATOR coNTnoL SYSTEM Herbert J. Hall, Hopewell Township, N. 3., assignor to Research Corporation, New York, N. Y., a corporation of New York Application March 4, 1954, Serial No. 414,003 7 Claims. (Cl. 1837) This invention relates to electrostatic precipitation and more particularly to systems for detecting and using the occurrence of transient sparks between precipitator electrodes to provide automatic voltage control so as to maintain a predetermined optimum sparking rate.

It is now well known in the electrostatic precipitator art that the most efficient operation is obtained in a given system by maintaining the voltage at a value which will cause a definite amount of sparking to occur between the electrodes. A lower voltage will not produce as complete precipitation, while a higher voltage will cause the sparking to increase to an intolerable degree. Various systems have been devised for sensing the sparking and controlling its rate, including that shown and described in applicants copending applications, Ser. No. 287,985, for Systems for Energizing Electrical Precipitators, filed May 15, 1952, now Patent No. 2,657,092, dated April 13, 1954; and Ser. No. 398,837, filed December 17, 1953, for Automatic Voltage Control of Electrostatic Precipitator with Saturable Reactor.

The primary object of this invention is to provide an improved means for sensing and utilizing precipitator sparking from the transient voltages disturbances occurring in the primary circuit of the H. V. power transformer of a precipitator, and for utilizing the same to control the precipitator voltage.

A further object is to provide a means for spark sensing which can conveniently be used to actuate electronic spark counting and integrating circuits such as that previously described in connection with the above-mentioned automatic voltage control systems for electrostatic precipitators.

A further object is to provide an improved precipitator spark sensing system which is connected in the low voltage circuit of the precipitator rectifier equipment directly at the main control panel. This obviates the necessity of long cable runs or special protective devices frequently used with spark sensing systems located in the high voltage portion of the precipitator electrical equipment.

Another object is to provide a spark sensing system which is extremely simple, reliable, and inexpensive, which uses no vacuum tubes and therefore requires no maintenance, and which can be easily installed in existing rectifier equipment.

Still another object is to provide a spark sensing system for precipitator which is highly sensitive to sparking incidents of extremely short duration, no more than a small fraction of a single cycle of the customary 60-cycle A. C. power supply, thereby making possible close control without the use of vacuum tube or other amplifiers in the sensing circuit.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing, in which:

Fig. 1 is a schematic circuit diagram of a two-section electrostatic precipitator, showing a preferred form of the invention;

2,752,000 Patented June 26, 1956 Figs. 2a to 2 inclusive, are a series of waveform graphs, related to the same time base, showing typical waveforms in different parts of the system due to the same sparking incidents;

Fig. 3 is a circuit diagram of a modified form of the sparking sensing device arranged for full-wave bridge rectifier operation;

Fig. 4 shows a modified form of the invention wherein the spark senser is connected in the secondary circuit of the precipitator;

Fig. 5 shows a typical waveform in the circuit of Fig. 4; and

Fig. 6 shows a complete automatic voltage control circuit using the new spark senser.

A conventional precipitator rectifier circuit is shown by way of example in Fig. 1. This comprises high voltage transformer 3 supplied from A. C. mains via a suitable voltage control device 1 and current limiting element such as resistor 2; the secondary is connected, for example, to a standard type rectifier circuit preferably one using vacuum tubes or similar rectifier elements including metallic types such as selenium; in Fig. l, a vacuum tube rectifier 4 is connected in a well-known manner to provide two negative, pulsating high voltage outputs energizing separate precipitator sections 5 and 6.

The spark senser 13 may consist of a small coupling transformer 7 having a primary winding 7a connected across the primary terminals of the main power transformer 3, and a center-tapped low voltage secondary winding 7b connected to a full-wave rectifier system employing elements 8a and 8b, each connected to a separate load resistor 9a, 9b. The two outputs from the spark senser may be connected to an electronic spark counting and integrating circuit such as that described in Patent 2,657,092, previously mentioned. The input circuit elements of such spark counter systems-switch 1%), coupling capacitance 11, and potentiometer 12are connected as shown.

The operation of the system may be explained by reference to typical circuit waveforms shown in Fig. 2. When the precipitator sparks, the voltage across the primary of transformer 3 drops sharply to a very low value near zero-limited only by the short circuit impedance characteristics of the high voltage rectifier circuit. This transient voltage disturbance is also impressed on the primary of coupling transformer 7 and apears in the rectified output of the spark senser across either resistor 9a or 9b, depending upon which of the precipitator sections, 5 or 6, sparked over. Assume, for example, that the transformer phase relationships are such that when precipitator section 5 sparks, the corresponding transient voltage disturbance appears across resistor 9b and when section 6 sparks, the primary voltage disturbances appear across resistor 9a. Thus, by means of switch 10, the input to the electronic spark counter and integrator may be a signal from either load resistor 9a or 912 separately, or the combined signals from both. Thus the sparks may be detected in each recipitator section separately or in both sections simultaneously.

Capacitance 11 and potentiometer 12 constitute a differentiating circuit so that the voltage across 12 is determined by the time rate of change of the input voltage and the voltage across the capacitance. Their values are so chosen that the normal line frequency loops in the waveforms across resistors 9a and 9b are virtually eliminated in the signal across 12 and only the sharp voltage pulse due to the steep wavefront resulting from the spark transient is transmitted to the grid of the amplifier tube in the electronic integrator. It will be noted that the rectifiers 8a and 8b are connected for negative output so that the transient voltage disturbance due to sparking will produce a positive going voltage spike across potentiometer 12. The arm of potentiometer 12 provides an amplifier gain control to actuate the electronic spark integrator and counter which may suitably be of the type shown in Patent 2,657,092, previously referred to. Positive rectification from the sensing transformer may also be used to obtain signals which could either be inverted in a subsequent stage or used directly, depending upon the design of the particular spark counter and integrating system.

As shown in Fig. 2a, a sparking event in precipitator 5 may produce a change as shown at time A in the voltage waveform across transformer 7, while a subsequent sparking event in precipitator 6 may produce a change as shown at time B. This produces the waveform changes shown respectively at Figs. 2b and 2c across resistors 91) and 9a respectively, which are differentiated by the resistance-capacitance combination at 11 and 512 to pro duce the sharp pulses shown in Figs. 2d, 2e and 2;, which represent respectively the connections of switch it in its various positions. It will be seen that the normal wa eform produces no pulse across resistor 12, while the steeper sparking waveform produces a pulse. The integrated elfect of these pulses in the spark rate integrator is used to actuate various types of control devices to regulate the voltage input to the precipitator and thus control the sparking at a desired rate.

Typical parameters for a practical system may be as follows:

Transformer '7400 volt primary, 12 volt centertapped secondary, a line frequency rating typically 60 cycles.

Diode rectifiers 3a and $b-various small selenium or germanium diodes may be used, for example, germanium diodes type INSZ. Vacuum tube diodes may also be used but are not preferred due to necessity of supplying filament heater voltage.

Load resistors 9a and 9bthese are not criticalcarbon resistors, for example, between about 2000 and 59% ohms are satisfactory.

Differentiating circuit input-capacitor 1125O tufand a 250,000 ohm potentiometer may be used. Other combinations may, of course, also be used.

The electronic spark counter and integrating circuits associated with the spark senser for purposes of instrumentation and precipitator automatic voltage control, based on maintaining an optimum average sparking rate in the precipitator, are described in the copending applications above referred to.

It will be obvious to those skilled in the art that other rectifying circuits for the spark senser could also be usedsuch as, for example, a. full-wave bridge without a center tapped winding for the secondary of transformer 7. The double output for detecting sparks from either or both precipitator sections in the conventional half-wave precipitator energization arrangement can, of course, be obtained. Fig. 3 shows a typical circuit arrangement, the details of which will be apparent from the figure without further description. It will also be apparent that a bridge rectifier arrangement of the type shown in Fig. 3 could be used at high level directly across the primary of the main transformer 3 of Fig. 1.

The spark senser described, which is responsive to voltage disturbances caused by precipitator sparking, can be connected at places in the precipitator rectifier circuit other than across the primary of the high voltage transformer. For example, the primary of transformer 7 may be connected across suitable taps on the primary of the high voltage transformer, or across a small tertiary signal winding if so desired. In addition, sparking may be sensed by the voltage transients across the primary resistor 2 or other current limiting element. For this purpose, rectifiers 8a and eb must be connected for a positive output for use with the electronic counter and integrator shown in the copending patent applications referred to above.

It is further possible to detect sparks from the precipitator rectifier secondary current return by connecting the senser across a resistance inserted in this return to ground. Typical circuits for accomplishing this are illustrated in Fig. 4. In Fig. 4, resistance 20 is inserted between rectifier tube 22 and ground and a similar resistor 24 between tube 26 and ground. These resistors may typically have a value from 50 to ohms. Leads 28 and 3t respectively lead from the high sides of the resistors to selector switch MB. The combined spark sensing d ferating device 7 may now be of a simpler form in that it need provide for pulses of one polarity only, since each half-wave of current in the secondary circuit through resistors 29 and 24 respectively is now rectified and of positive polarity. This is generally not a preferred method, however, because of necessity of long cable runs required in case the precipitator high voltage transformer is located some distance away from the main control panel where control equipment for automatic voltage regulation, for example, would be located. These various methods of spark sensing with the voltage responsive device disclosed herein are mentioned to show the simplicity and versatility of the system. It will be noted that in sensing sparks from the high voltage rectifier secondary current return, the coupling transformer can be eliminated if desired and the rectified voltage across the series resistors 29, 24 fed directly to the differentiating circuit in the electronic spark counter. This method provides an extremely simple system by using the normal high voltage rectifiers to perform the function of auxiliary rectifiers to the spark senser when the latter is connected in the alternating voltage primary circuit.

Fig. 5 shows a typical waveform across resistors 20 and/or 24. A normal half-cycle of voltage is shown at time T, and a typical waveform due to a sparking incident at time 2, followed again by the normal condition at time 3. The transient voltage spark at time 2 is sensed by the system as in Fig. 1, while the normal wave shape produces no output from the senser.

Although a number of spark sensing arrangements have been shown or discussed, the preferred system is that shown in Fig. l. in addition to the convenience of sensing in the primary circuit, the voltage transients across the high voltage transformer primary are much larger in magnitude and have much steeper wavefronts than those occurring across the primary series resistance or other current limiting means. The large voltage pulse amplitudes available in the system of Fig. 1 even at very low precipitator load currents, eliminates the necessity of high gain, sensitive amplifiers in the electronic spark integrator circuits and renders the spark sensing system independent of the precipitator load current which in normal operation may vary over wide limits. It should be emphasized that the primary and secondary voltage ratings for the coupling transformer as given on page 5 are by way of example only. The primary rating used in any particular case would depend on the design rating of the high voltage power transf0rmer400 volts is commonly used for precipitator rectifier equipment, but other operating voltages may also be used. The secondary voltage rating of coupling transformer 7 may be any value consistent with the overall design requirements of the electronic spark rate counter and integrator or other system employed. As already pointed out, the spark senser can provide large amplitude pulses if desired.

Fig. 6 shows a complete automatic voltage control system embodying the spark sensing principle above described. ln this case, the voltage control device 1 is represented as a variable auto-transformer having a variable control contact Eti, the position of which controls the input voltage to the transformer 3' and thus controls the precipitator electrode voltage. The spark senser 7" is connected as in Fig. l, and its output is supplied to an electronic spark rate integrator 31 which may be similar to that shown in either of my prior copending applications,

Ser. Nos. 287,985 or 398,837, previously referred to. The output of the integrator may be supplied to a differential magnetic amplifier 32 which in turn controls a reversible servo-motor 34, as shown in Ser. No. 398,837, which is used, through suitable reduction gearing indicated schematically at 36, to control the position of variable control contact 39. As the components of the system are fully described in the copending applications, the details thereof are not shown in the present application. It will be apparent that the spark senser shown lends itself to use in various types of voltage control systems and is not restricted to use with the particular system here shown by way of example.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. An electrical precipitator system comprising a highvoltage step-up transformer supplied from an alternating current supply source, high-voltage rectifying means for the output of said transformer, precipitator electrodes supplied by the output of said rectifier, and means responsive to individual sparking events at said electrodes comprising circuit means for deriving a voltage related to the instantaneous voltage across the primary of said transformer, and further circuit means for producing a pulse output upon the occurrence of a voltage transient disturbance in the transformer primary due to a sparking event across said precipitator electrodes, said last means comprising a differentiating circuit of short time-constant relative to the power supply frequency for producing a pulse output only upon the occurrence of a sharp transient voltage at its input.

2. In combination with an electrical precipitator system comprising a high voltage step-up power transformer, high voltage rectifying means for the output of said transformer, and precipitator electrodes connected to the output of said rectifier means; means responsive to individual sparking events at said electrodes comprising a coupling transformer having its primary winding connected across the primary Winding of the power transformer, a rectifier circuit including two grounded resistance units connected respectively one across each half-wave output terminal of a full-wave bridge rectifier connected to the secondary of said coupling transformer.

3. In combination with an electrical precipitator system comprising a high voltage step-up power transformer, high voltage rectifying means for the output of said transformer, and precipitator electrodes connected to the output of said rectifying means; means responsive to individual sparking events at said electrodes comprising a coupling transformer having its primary winding connected across the primary winding of the power transformer, a grounded center-tapped secondary, a resistance unit and rectifier connected in series between the center tap and each secondary terminal of the transformer for fuiLw-are operation, a grounded output circuit, and a selector switch for selectively connecting said output circuit to the ungrounded side of either resistance unit or across both resistance units in parallel.

4. The invention according to claim 3, said output circuit comprising a differentiating circuit.

5. The invention according to claim 4, said output circuit comprising a capacitor and a resistance unit, and a utilization circuit connected across said resistance unit.

6. The invention according to claim 5, said resistance unit being variable.

7. An electrical precipitator system comprising a high voltage step-up transformer supplied from an alternating current supply source, high voltage rectifying means for the output of said transformer, precipitator electrodes supplied by the output of said rectifier; means responsive to individual sparking events at said electrodes comprising circuit means for deriving a voltage related to the instantaneous voltage across the primary of said transformer, and differentiating circuit means for producing a pulse output upon the occurrence of a voltage transient disturbance in the transformer primary due to a sparking event across said precipitator electrodes, the constants of said differentiating circuit means being of such value as to produce no appreciable output in response to the ordinary power supply frequency, a pulse integrating circuit for producing an electrical output related to the integrated pulse input rate, and a voltage control device for controlling the input to said high voltage transformer in accordance with the value of said electrical output.

References Cited in the file of this patent UNITED STATES PATENTS 1,976,569 Levy Oct. 9, 1934 2,297,740 Brown Oct. 6, 1942 2,666,496 Willison Jan. 19, 1954 2,675,092 Hall Apr. 13, 1954 

